Proton therapy is one of the best methods available for treating cancer. While the value of Monte Carlo (MC) simulation for proton therapy is well established, there exists no software that combines ease-of-use with the level of flexibility needed for demanding clinical and research applications. The release of the BEAM/DOSXYZ MC in 1995 effectively addressed a similar barrier for conventional therapy, releasing a flood of valuable research projects and clinical applications, but this application does not work for proton therapy. A fast, easy-to-use BEAM/DOSXYZ for Protons or PBeam will unlock great potential for MC in proton therapy. It will enable researchers and clinical physicists to accurately determine dosimetric and radiobiological aspects of proton beams for individual patients, accelerate design of new proton treatment planning and delivery systems, facilitate design of detectors for dose measurement and imaging devices for image- guided radiotherapy, and provide clinicians with the capability to exploit the power of MC simulation in clinical practice, including the creation of treatment plans for individual patients with the accuracy and functionality required to optimize treatment. We will provide PBeam free of charge, for all common operating systems, with online training materials, minimizing barriers to use. PUBLIC HEALTH RELEVANCE: While the value of Monte Carlo simulation for proton therapy is well established, there exists no software for this purpose that combines ease-of-use with the level of flexibility needed for demanding clinical and research applications. Consequently, this valuable calculation technique remains underutilized, difficult to apply and restricted to use by highly experienced software experts. With today's climate of increasing numbers of proton facilities and proton-related research, a fast and easy-to-use PBeam will unlock great potential for Monte Carlo in proton therapy, helping researchers study dosimetric and radiobiological aspects of proton beams, helping them improve proton delivery, treatment planning and validation and helping clinicians exploit the power of Monte Carlo simulation in routine clinical practice.